The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.
VCSELs, and particularly VCSEL arrays, emitting in the range of 50 mW to 10 W of optical power are important technology for applications within a variety of markets, including but not limited to, the consumer, industrial, automotive, and medical industries. Example applications include, but are not limited to, illumination for security cameras, illumination for sensors such as three-dimensional (3D) cameras or gesture recognition systems, medical imaging systems, light therapy systems, or medical sensing systems such as those requiring deep penetration into tissue. In such optical sensing and illumination applications as well as other applications, VCSELs and VCSEL arrays offer several benefits, as will be described in further detail herein, including but not limited to, power efficiency, narrow spectral width, narrow beam divergence, and significant packaging flexibility.
Indeed, for VCSELs and VCSEL arrays, power conversion efficiency (PCE) of greater than 40% can be achieved at wavelengths in at least the 800-1000 nm range. PCE may be defined as the ratio of optical power emitted from a laser(s), such as a VCSEL or VCSEL array, divided by the electrical power used to drive the laser(s). While VCSEL PCE, alone, is fairly comparable to that for some of the most efficient light-emitting diodes (LEDs) currently available, when spectral width and beam divergence are considered, there are significant efficiency benefits to VCSELs over LEDs.
For example, VCSEL arrays generally have a spectral width of approximately 1 nm. This allows the use of filters for a photodetector or camera in order to reduce the noise associated with background radiation. For comparison, a LED typically has a spectral linewidth of 20-50 nm, resulting in the rejection of much of the light by such a filter, and hence reducing the effective PCE of the LED. In addition, the wavelength of a VCSEL is less sensitive to temperature, increasing only around 0.06 nm per 1° Celsius. The VCSEL rate of wavelength shift with temperature is four times less than in a LED.
Also for example, the angular beam divergence of a VCSEL is typically 10-30 degrees full width half maximum (FWHM), whereas the output beam of a LED is Lambertian, filling the full hemisphere. This means that generally all, if not all, of the light of a VCSEL can be collected using various optical elements, such as lenses for a collimated or focused beam profile, diffusers for a wide beam (40-90 degrees or more) profile, or a diffractive optical element to generate a pattern of spots or lines. Due to the wide beam angle of a LED, it can be difficult to collect all or nearly all of the light (leading to further degradation of the PCE), and also difficult to direct the light as precisely as is possible with a VCSEL.
The vertically emitting nature of a VCSEL also gives it much more packaging flexibility than a conventional laser, and opens up the door to the use of the wide range of packages available for LEDs or semiconductor integrated circuits (ICs). In addition to integrating multiple VCSELs on the same chip, as will be described in further detail below, one can package VCSELs or VCSEL arrays with photodetectors or optical elements. Plastic or ceramic surface mount packaging or chip-on-board options are also available to the VCSEL.
VCSEL geometry traditionally limits the amount of optical power a VCSEL can provide. To illustrate the issue, FIG. 1 is a diagram of the cross-section of a typical VCSEL 100, and includes general structural elements and components that may be utilized, as an example, for VCSEL and VCSEL array embodiments disclosed herein. In general, epitaxial layers of a VCSEL may typically be formed on a substrate material 102, such as a GaAs substrate. On the substrate 102, single crystal quarter wavelength thick semiconductor layers may be grown to form mirrors (e.g., n- and p-distributed Bragg reflectors (DBRs)) around a quantum well based active region to create a laser cavity in the vertical direction. For example, on the substrate 102, first mirror layers 104 may be grown, such as but not limited to layers forming an AlGaAs n-DBR. A spacer 106, such as but not limited to an AlGaAs or AlGaInP spacer, may be formed over the first mirror layers 104. Then a quantum well based active region 108, such as but not limited to an AlGaInP/GaInP or GaAs/AlGaAs multiple quantum well (MQW) active region, may be formed, along with another spacer layer 110, such as but not limited to an AlGaAs or AlGaInP spacer. Over that, second mirror layers 112 may be grown, such as but not limited to layers forming an AlGaAs p-DBR, over which a current spreader/cap layer 114 may be formed, such as but not limited to, an AlGaAs/GaAs current spreader/cap layer. A contacting metal layer 116 may be formed over the cap layer 114, leaving an aperture of desired diameter generally centered over the axis of the VCSEL. In some embodiments, a dielectric cap 120 may be formed within the aperture 118. As will be explained in more detail below with specific reference to certain embodiments of the present disclosure, a mesa 122 may be formed by etching down the epitaxial structure of the VCSEL to expose a higher aluminum containing layer or layers 124 for oxidation. The oxidation process leaves an electrically conductive aperture 126 in the oxidized layer or layers that is generally aligned with the aperture 118 defined by the contacting metal layer 116, providing confinement of current to the middle of the VCSEL 100. More specific details regarding VCSEL structure and fabrication as well as additional VCSEL embodiments and methods for making and using VCSELs are disclosed, for example, in: U.S. Pat. No. 8,249,121, titled “Push-Pull Modulated Coupled Vertical-Cavity Surface-Emitting Lasers and Method;” U.S. Pat. No. 8,494,018, titled “Direct Modulated Modified Vertical-Cavity Surface-Emitting Lasers and Method;” U.S. Pat. No. 8,660,161, titled “Push-Pull Modulated Coupled Vertical-Cavity Surface-Emitting Lasers and Method;” U.S. Pat. No. 8,989,230, titled “Method and Apparatus Including Movable-Mirror MEMS-Tuned Surface-Emitting Lasers;” U.S. Pat. No. 9,088,134, titled “Method and Apparatus Including Improved Vertical-Cavity Surface-Emitting Lasers;” U.S. Reissue Pat. No. RE41,738, titled “Red Light Laser;” and U.S. Publ. No. 2015/0380901, titled “Method and Apparatus Including Improved Vertical-Cavity Surface-Emitting Lasers;” of which the contents of each are hereby incorporated by reference herein in their entirety. Without being limited to solely the VCSELs described in any one of the foregoing patents or patent applications, VCSELs suitable for various embodiments of the present disclosure or suitably modifiable according to the present disclosure include the VCSELs disclosed in the foregoing patents or patent applications, including any discussion of prior art VCSELs therein, as well as VCSELs disclosed in any of the prior art references cited during examination of any of the foregoing patents or patent applications. More generally, unless specifically or expressly described otherwise, any VCSEL now known or later developed may be suitable for various embodiments of the present disclosure or suitably modifiable according to the present disclosure.
For efficient operation of a VCSEL, a method for providing current confinement in the lateral direction (achieved with the electrically insulating oxidation layer shown) to force current flow through the center of the device is often required. The metal contact 116 on the surface of the device may provide a means for injecting current into the VCSEL. As described above, the metal contact 116 should have an opening or aperture 118 in order to allow the light to leave the VCSEL. There is a limit to how far current can be spread efficiently across this aperture, and hence there is a limit to the lateral extent of the laser, and in turn, the maximum power that can be emitted from a single aperture. A solution to this, for applications requiring more power, is to create multiple VCSELs on a chip that operate together in parallel. In such an approach, the total output power can be scaled simply by scaling the number of VCSEL devices or apertures. FIG. 2 illustrates an example layout for a VCSEL die or chip 200 with, for example, one hundred eleven (111) VCSEL devices/apertures 202. A common metal layer 204 on the top surface of the chip 200 (or similar contact mechanism) may contact the anode of each VCSEL device 202 simultaneously, and a common cathode contact (or similar contact mechanism) may be made on the backside of the chip, allowing all the VCSEL devices to be driven in parallel.
An array approach not only solves the technical issue of emitting more optical power, but also provides important advantages. For example, a conventional single coherent laser source results in speckle, which adds noise. However, as will be explained in more detail below with respect to embodiments of the present disclosure, speckle contrast can be reduced through the use of an array of lasers which are mutually incoherent with each other.
Another advantage or benefit is that of improved eye safety. An extended source is generally more eye safe than a point source emitting the same amount of power. Still another advantage or benefit is the ability to better manage thermal heat dissipation by spreading the emission area over a larger substrate area.
Requirements for an optical source typically depend upon the application and the sensing mechanism used. For example, illumination for night vision cameras may involve simply turning on the light source to form constant uniform illumination over a wide angle which is reflected back to the camera. However, additional illumination schemes can provide more information, including but not limited to, three-dimensional (3D) information. FIGS. 3A-C illustrate example sensing mechanisms—structured lighting, time-of-flight, and modulated phase shift—used to gather information in three dimensions. As illustrated in FIG. 3A, in structured lighting, a pattern (e.g., dots, lines, more complex patterns, etc.) 302 may be imposed upon the light source 304, and then one or more cameras 306 are used to detect distortion in the structure of the light to estimate distance. As conceptually illustrated in FIG. 3B, in a time-of-flight approach, a time-gated camera may be used to measure the roundtrip flight time of a light pulse. As graphically illustrated in FIG. 3C, in the case of modulated phase shift, an amplitude modulation may be imposed upon the emitted light, and the phase shift between the emitted beam and reflected beam may be recorded and used to estimate the distance travelled.
Typically, requirements of an optical light source for any given application may include consideration of one or more of the following:
1. Optical output power: Sufficient power is required for illumination of the area of interest. This can range from tens of milliwatts optical power, such as for a sensing range of a generally a few centimeters, to hundreds of milliwatts, such as for games or sensing of generally a meter or two or so, to ten watts, such as for collision avoidance systems, and kilowatts of total power, such as for a self-driving car.
2. Power efficiency: Particularly for mobile consumer devices, a high efficiency in converting electrical to optical power is desirable and advantageous.
3. Wavelength: For many applications, including most consumer, security, and automotive applications, it may be preferable that the illumination be unobtrusive to the human eye, and often in the infrared region. On the other hand, low cost silicon photodetectors or cameras limit the wavelength on the long end of the spectrum. For such applications, a desirable range therefore, may be generally around or between 800-900 nm. However, some industrial applications may prefer a visible source for the purpose of aligning a sensor, and some medical applications may rely on absorption spectra of tissue, or materials with sensitivity in the visible regime, primarily around 650-700 nm.
4. Spectral width and stability: The presence of background radiation, such as sunlight, can degrade the signal-to-noise ratio of a sensor or camera. This can be compensated with a spectral filter on the detector or camera, but implementing this without a loss of efficiency often requires a light source with a narrow and stable spectrum.
5. Modulation rate or pulse width: For sensors based, for example, upon time of flight or a modulation phase shift, the achievable pulse width or modulation rate of the optical source can determine the spatial resolution in the third dimension.
6. Beam divergence: A wide variety of beam divergences might be specified, depending upon whether the sensor is targeting a particular spot or direction, or a relatively larger area.
7. Packaging: The package provides the electrical and optical interface to the optical source. It may incorporate an optical element that helps to control the beam profile, and may generate a structured lighting pattern. Particularly for mobile devices or other small devices, the overall packaging would desirably be as compact as possible.
In view of the foregoing, there is a need in the art for VCSELs or VCSEL arrays, or configurations thereof, that enhance performance or functionality for use, for a non-limiting example, as illumination sources for cameras and sensors.